Novel hydroxypyridinone derivatives containing an oxime ether moiety: Synthesis, inhibition on mushroom tyrosinase and application in anti-browning of fresh-cut apples.

A range of hydroxypyridinone derivatives were synthesized starting from kojic acid. Among them, 10 and 11 were found to possess the strongest inhibitory effect on monophenolase activity of mushroom tyrosinase, having IC50 values of 2.04 and 1.60µM, respectively. The IC50 values of 10 and 11 for the inhibition of diphenolase activity of mushroom tyrosinase were determined as 13.89 and 7.99µM, respectively. Investigation of the inhibitory mechanism of these two compounds indicated that the inhibition was reversible and of a competitive-uncompetitive mixed type. The KI and KIS values of 10 were determined to be 24.84 and 32.54µM, respectively, and the corresponding values for 11 being 18.07 and 21.34µM, respectively. The effect of 11 on the browning process of fresh-cut apples was evaluated by measuring the color change and browning index. The results indicated that 11 had a significant effect on controlling the browning of fresh-cut apple slices.

Carboxymethylated degraded polysaccharides from Enteromorpha prolifera: Preparation and in vitro antioxidant activity.

In order to improve the bioactivities of the polysaccharide from Enteromorpha prolifera (PE), crude PE (Mw 1400kDa) was degraded to low molecular weight polysaccharide (44kDa) in the presence of hydrogen peroxide/ascorbic acid, followed by carboxymethylation. The reaction conditions for carboxymethylation of degraded polysaccharide (DPE) were optimized by Response Surface Methodology. The carboxymethyled degraded polysaccharide (CDPE) obtained under optimized conditions, with a degree of carboxymethylation of 0.849, was characterized by FT-IR and (13)C NMR. The molecular weight of CDPE was measured to be 53.7kDa. CDPE was evaluated for its antioxidant activity by determining the ability to scavenge 1,1-diphenyl-2-picrylhydrazyl (DPPH), hydroxyl and superoxide anion radicals, and by determining the ferric reducing power. The antioxidant activity of CDPE was found to be greatly improved in comparison with degraded polysaccharide (DPE) and crude polysaccharide from Enteromorpha prolifera (PE).

Design and synthesis of novel hydroxypyridinone derivatives as potential tyrosinase inhibitors.

Two groups of novel hydroxypyridinone derivatives 6(a-e) and 12(a-c), were designed as potential tyrosinase inhibitors, and synthesized using kojic acid as a starting material. The tyrosinase inhibitory activity of these two groups was demonstrated to be potent, especially compounds 6e and 12a, whose IC50 values for monophenolase activity were 1.95µM and 2.79µM, respectively. Both of these values are lower than that of kojic acid (IC50=12.50µM). Compounds 6e and 12a were investigated for the inhibitory effect on diphenolase activity. The results showed that the inhibitory mechanism of these two compounds was reversible and that the inhibitory type was a competitive-uncompetitive mixed-type. The values of IC50 of 6e and 12a on the diphenolase activity of tyrosinase were determined to be 8.97µM and 26.20µM, respectively. The inhibitory constants (KI and KIS) of 6e were determined as 17.17µM and 22.09µM, respectively; and the KI and KIS values of 12a were 34.41µM and 79.02µM, respectively. Compound 6e showed a greater ability to reduce copper and a stronger copper chelating ability than kojic acid.